Mufflers, for example, and other exhaust components extending from an engine include exhaust component covers referred to as noise and heat insulation covers installed particularly in regions prone to sound and heat, such as is disclosed in Japanese Patent Application Laid-Open Publication No. 2002-87078 (JP 2002-087078 A), for example. The exhaust component cover disclosed in JP 2002-87078 A is shown in FIG. 14 hereof.
Referring to FIG. 14, an exhaust component cover 100 is manufactured by joining an internal plate 103 to an external plate 101 via aluminum foil 102, bending one end of the external plate 101 in the direction of the internal plate 103, and then integrally press-forming the external plate 101, the aluminum foil 102, and the internal plate 103 together.
The exhaust component cover 100 manufactured in this manner covers regions of the exhaust component prone to sounds, and muffles noise produced by the exhaust component.
While the engine is operating, the vibration of the engine is transmitted to the exhaust component, and the vibration is further transmitted to the exhaust component cover. This vibration causes self-excited vibration in the exhaust component cover, and also causes so-called radiant noise.
The natural frequency fn of the exhaust component cover 100 is expressed by the equation fn=1/(2π)×(k/m)0.5. The letter k denotes a spring constant whose numerical value differs depending on the material, and the letter m denotes mass.
When the external plate 101 and the internal plate 103 have different thicknesses, the external plate 101 and the internal plate 103 also have different masses in; therefore, the external plate 101 and the internal plate 103 will have different natural frequencies fn, and the vibration can be damped.
However, in order to damp the vibration of the exhaust component cover 100 described above, an external plate 101 and an internal plate 103 of different thicknesses must be prepared, increasing both the weight and number of components.